Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of the positive silver image, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith a soluble silver complex is obtained by reaction of a silver halide solvent with the unexposed and the undeveloped silver halide of said emulsion. Preferably, the photosensitive silver halide emulsion is developed with a processing composition in a viscous condition which is spread between the photosensitive element comprising the silver halide emulsion and a print receiving element comprising a suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion and, substantially contemporaneous therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate complex, with undeveloped silver halide. This soluble silver complex is, at least in part, transported in the direction of the print receiving element and the silver ion thereof is largely reduced to silver metal and precipitated in the silver precipitating element to form a positive image thereon. Procedures of this type are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See also Edwin H. Land; One Step Photography; Photographic Journal, Section A, pages 7-15, January, 1950.
Additive color reproduction may be obtained by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color such as red, or green, or blue, and by viewing the reversed or positive silver image formed by transfer to a transparent print-receiving element through the same or a similar screen which is suitably registered with the reversed positive image carried by the print-receiving layer.
As examples of suitable film structures for employment in additive color photography mention may be made of the U.S. Pat. Nos. 3,861,885, 2,726,154, 2,944,894, 3,536,488, 3,615,427, 3,615,428, 3,615,429, 3,615,426, and 3,894,871.
The utility of such film units wherein the silver halide layer remains an integral portion of the film unit subsequent to positive image formation is achieved by employing as the image-receiving element a layer which provides an unusually effective silver precipitating environment which causes the silver deposited therein to possess extraordinarily high covering power in comparison with negative silver developed in the silver halide layer.
The above-mentioned integral film unit, particularly those of the above-disclosed U.S. patents, are particularly desirable for employment as cine film for motion picture projection, for example, such as the cine film system described in U.S. Pat. No. 3,615,427 issued Oct. 26, 1971. Processing of such film units as well as the specific composition of the processing composition is detailed in the aforementioned patents.
It is known in the art that various inorganic materials may be disposed in the photosensitive silver halide emulsion layer for the purpose of providing an increase in film speed. For example, British Pat. No. 504,283 issued Apr. 21, 1939 is directed to a silver halide emulsion layer having incorporated therein pigment particles. Among the pigment materials suitable for use as a diffusing medium, mention is made of titanium dioxide, zinc oxide, magnesium oxide, zirconium oxide, silicates, and the like. It is also stated that the pigments must be substantially unaffected by the solutions normally used in processing the silver halide emulsion layers.
U.S. Pat. No. 3,462,265 issued Aug. 19, 1969 discloses and claims the utilization of particulate aluminum especially in flake form uniformly disposed in a layer of photosensitive silver halide. One advantage that accrues from the employment of said aluminum flake is an increase in the film speed as a result of the reflection of part of the incident light during the exposure by the aluminum flake thereby producing a stronger latent image.
Employment of some of the above-mentioned materials in photosensitive silver halide layers in film units of the projection type, such as the integral film units set forth in the above-indicated U.S. patents, while increasing the film speed of the emulsions, is detrimental to the finished product since some of the materials remain in the emulsion, associated with the positive image, thus blocking or diffusing some the light incident on the film unit necessary for projection of the image. In effect, therefore, many of such speed increasing materials cannot be effectively employed in transparency or other projection type film units.
The coating art has employed for a number of years microscopic air bubbles or microvoids in organic binder materials as an opacifying material to provide coatings without pigment or with a minimum of pigment. Such microvoids find employment in pressure and heat sensitive films to produce inkless printable surfaces, duplicating paper, chart recording paper, and the like. Microvoid structures have been produced in a number of ways, for example, by freeze drying, extraction, phase incompatibility, imperfect packing and the use of solvent and non-solvent combinations for a polymer in wet coating formulations. More details may be found on the preparation and use of microvoids in the Journal of Paint Technology, Vol. 43, No. 559, August, 1971, pages 48 to 53, and by Md. Eng. Chem. Prod. Res. Develop, Vol. 13, No. 1, 1974, pages 30 to 33. The latter article includes a summary of patent literature relating to microvoids.